How to insert output of command into string in MikroTik RouterOS (scripting)

If you have a MikroTik RouterOS command such as

/tool e-mail send [email protected] subject="MikroTik test E-Mail"

you can insert the output of a command such as

/system identity get name

into it by using the $[...] syntax:

/tool e-mail send [email protected] subject="My identity is $[/system identity get name]"